With superposition coding at the transmitter and successive interference cancellation (SIC) at the receiver,
NOMA can achieve higher spectral efficiency than OMA.
Moreover, NOMA can serve multiple users
at the same resource (e.g., time/frequency) by exploiting the
user differences in the power domain [1]}, [2]}.
This power domain feature provides
rich opportunities for NOMA to support massive connectivity
and meet the users' diverse quality of service.
